Why sleep?

Humans spend about one-third of their life asleep. Yet science knows remarkably little about why we need to sleep, what happens when we sleep, or why we feel restored by a good sleep.

We do know that sleep deprivation has serious consequences. This is because a lot of activity takes place in the body and the brain during sleep. For example, when it’s dark, our bodies produce extra melatonin to grow and repair cells and muscle tissue. Our bodies start to feel sleepy in response to the melatonin.

Sleep is necessary for the brain and body to develop. This is one of the reasons why very young children spend so much time sleeping.

In adolescence and the adult years, active sleep is called rapid eye movement (REM) sleep. Active sleep is a form of light sleep.

In quiet sleep, babies lie still and breathe deeply. They’ll occasionally jerk or ‘startle’. Quiet sleep is similar to non-REM sleep in older children and grown-ups. It can be light or deep.

Light and deep sleep
Light sleep can be very light – sometimes you might not even realise you’re asleep. We can be easily woken from light sleep.

Someone in deep sleep is hard to wake. Deep sleep is a lot more peaceful and restful. It’s thought that deep sleep is the time when we grow and heal. Someone in deep sleep might feel quite drowsy if woken.

Sleep cycles

In the course of one night, grown-ups and children move through repeating cycles of quiet and active sleep.

Typically, more of our sleep in the early part of the night is quiet sleep (about 80%). Then, about halfway through our normal-length sleep, our sleep cycle flips. By morning, about 80% of our sleep is active sleep.

This is why it’s easier to be disturbed towards the end of a sleep.

Sleep cycles in babies and children
Cycles of quiet and active sleep last 30-50 minutes in babies, then gradually increase in length across childhood.

Some babies and children fall deeply asleep very quickly. Others sleep lightly, fidgeting and muttering for up to 20 minutes, before getting into deep sleep.

Children usually wake briefly at the end of each sleep cycle. This is a normal part of healthy sleep – all children do it. Some children call out when they wake and need help settling again, but independent sleepers put themselves back to sleep. Not all parents hear their children when they wake up.

Sleep cycles in adolescence and adulthood
In adolescence and during the adult years, each cycle of active and quiet sleep lasts about 90 minutes.

Each cycle ends in a brief awakening, and these can happen several times throughout the night. These awakenings don’t normally disrupt our sleep, and we usually aren’t even aware of them. If things haven’t changed, we’ll normally go straight back to sleep – but if things have changed around us, we’re uncomfortable or anxious, our pillow is missing, or we’re disturbed by a noise, we might wake fully.

How sleep cycles change over time

The amount of time we spend in each type of sleep varies depending on our age.

At birth, full-term infants spend about half their sleeping time in active sleep. Each sleep cycle lasts only 40 minutes (compared to 90 minutes for a grown-up). This means that, biologically, infants are programmed to sleep more lightly and have more awakenings than grown-ups.

The amount of active sleep in our sleep cycle decreases with age. By three years of age, 33% of sleep is active.

By the time we’re around 13 years old, only about 20% of our sleeping time is active sleep.

When we sleep

It takes time to consolidate most of our sleep into the night-time. Babies and children vary in their sleep habits and sleep requirements, just like adults. It can be a good idea to have routine sleep times to stop your child getting overtired or sleepy during the day – see what works best for your child.

Under six months
Newborns sleep on and off through the day and night.

Babies aged 3-6 months might start moving towards a pattern of 2-3 daytime sleeps of up to two hours each. They might still wake at least once at night.

From 6-12 months
From about six months, babies have their longest sleep at night.

Between six and 12 months, most babies are in bed between 6 pm and 10 pm. They usually take less than 30 minutes to get to sleep (but about 10% of babies take longer).

Most babies can sleep for a period of six hours or more at night and are waking less. About 60% will wake only once during the night and need a grown-up to settle them back to sleep. About one in 10 will call out 3-4 times a night. More than a third of parents report problems with their baby’s sleep at this age.

Around 85-90% of infants aged 6-12 months are still having daytime naps. These naps usually last 1-2 hours. Some infants will sleep longer, but up to a quarter nap for less than an hour.

Waking at night is partly related to the child’s worry about being separated from a parent or parents. Overcoming this worry is normal and necessary for all children as a step towards becoming more independent at night.

From 12 months
From this age, children tend to sleep better. Some toddlers start to resist going to sleep at night, preferring to stay up with the family – this is the most common sleep problem reported by parents. It peaks around 18 months and improves with age.

Less than 5% of two-year-olds wake three or more times overnight.

From 3 years
Children aged 3-5 years need around 11-13 hours of sleep a night. Some might also have a day nap that lasts for about an hour.

Children aged 6-9 need 10-11 hours sleep a night. They’re usually tired after school and might look forward to bedtime from about 7.30 pm.

From 10 years
Children entering puberty generally need about 9¼ hours of sleep a night to maintain the best level of alertness during the day.

Changes to the internal body clock or circadian rhythm during adolescence means it’s normal for teenagers to want to go to bed later at night – often around 11 pm or later – then get up later in the morning.

Over 90% of adolescent children don’t get the recommended amount of sleep on school nights. Getting enough good-quality sleep is important during this period, because sleep is vital for thinking, learning and concentration skills.

Parents can help children get enough sleep by encouraging a set bed time on school nights, then allowing kids to stay up a bit later on the weekends. To prevent Monday morning blues and early week tiredness, it can be a good idea for teenagers not to get up too late on weekends.

Daytime sleepiness, difficulty getting to sleep or staying asleep, or finding it hard to wake up in time for school are all common experiences in adolescence. Over 45% of teenagers report daytime sleepiness at least once a week.